Pipe Pressure Drop

Calculate friction head loss and fluid velocity in water pipes using the empirical Hazen-Williams equation.

Flow Rate (Q)

Pipe Length (L)

meters

Inner Diameter (D)

Pipe Material (C Factor)

Results (Hazen-Williams)

0.00

m.c.a. (Meters of Head)

0.00

Bar (Pressure Drop)

0.00

Fluid Velocity (m/s)

Warning: The velocity exceeds 2.5 m/s. It is recommended to increase the pipe diameter to prevent water hammer and excessive noise.

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The Physics of Fluid Friction

In the design of pumping systems, irrigation networks, or HVAC circuits, fluid (usually water) encounters physical resistance as it travels through the interior of a pipe. This resistance, or friction, depends directly on the pipe's internal roughness and the velocity at which the fluid is moving.

As water rubs against the inner walls, it loses kinetic energy. This lost energy mathematically translates into a pressure drop, also known in engineering terminology as Friction Head Loss.

Do you already know the Total Dynamic Head (TDH)? Size the motor now with the Water Pump Power Calculator.

The Empirical Hazen-Williams Equation

While there are several methods to calculate fluid friction (such as the complex Darcy-Weisbach equation), for the flow of water at ordinary temperatures in closed conduits, the industry has universally adopted the empirical Hazen-Williams formula due to its extreme reliability and parametric simplicity.

h_f = 10.67 × L × ( Q / C )^1.852 / D^4.87

Where the components of the formula in the Metric System are:

h_f = Head loss obtained in meters of water column (m.c.a.)
L = Real length of the straight pipe in meters (m)
Q = Volumetric flow rate circulating in cubic meters per second (m³/s)
D = Inner diameter of the pipe in meters (m)
C = Hazen-Williams roughness coefficient (dimensionless). It is inverse to roughness; the smoother the pipe, the higher the C value (e.g., Smooth PVC has C=150, while rusted steel can have C=90).

Controlling Fluid Velocity

An engineer must not only look at the pressure drop but also closely monitor the linear velocity of the water inside the tube. Our calculator includes this critical parameter highlighted in blue. The continuity equation dictates that:

Velocity (V) = Flow Rate (Q) / Cross-Sectional Area (A)

Normative design rules: It is highly recommended that water velocity is maintained between 1.0 m/s and 2.5 m/s (approx. 3 to 8 ft/s). Lower velocities cause the sedimentation of heavy particles (fouling the pipe), while velocities exceeding 2.5 m/s generate excessive noise due to turbulence, drastically increase pressure drop, and cause the dangerous phenomenon known as Water Hammer when valves are closed suddenly.